States of matter: same particles, different distances

The problem

Block the tip of a syringe full of air and push. The plunger moves. Do the same with water and it hardly moves at all. Same syringe, same push. What is different inside?

Your book calls anything other than energy that carries weight and occupies volume matter. A state of matter is one of the distinct forms it can take. In everyday life we meet four: solid, liquid, gas and plasma.

The three primary states differ in three ways: how strongly the particles attract each other, how they are arranged, and how far apart they are (Fig 1.1).

Solids, liquids and gases differ in the strength of the forces between their particles, the arrangement of the particles and the distance between them. The farther apart the particles, the easier the matter is to compress and the lower its density.

Key terms

Plasma

Particles with very high kinetic energy: electrons, ions and photons. Rare on Earth but, as your book says, most matter in the rest of the universe is in this state.

Supercritical fluid

A highly compressed state with properties of both gases and liquids. Some reactions that fail in ordinary solvents can be carried out in supercritical carbon dioxide.

Liquid crystal

Its properties lie between those of ordinary liquids and crystalline solids. Used in display devices such as computer monitors, clocks, watches and navigation systems.

Exotic state

States of matter that are not commonly encountered. Your book names dark matter, Bose-Einstein condensate, nuclear matter and quantum spin liquid.

Short questions with model answers

1. Q1. Explain why air in a closed syringe can be squeezed but water hardly can.

   • Air is a gas: its molecules are very far apart.
   • Water is a liquid: its molecules are close together.

   Gas molecules are far apart, so gases are easily compressed; liquid molecules are close together, so liquids are not easily compressed.

2. Q2. Arrange solid, liquid and gas from highest to lowest density and give the reason.

   • solid > liquid > gas
   • Closer particles → more mass in the same volume.

   Solid, then liquid, then gas: the particles are closest in solids and farthest apart in gases.

3. Q3. Why are supercritical fluids important?

   • They show properties of both gases and liquids.
   • Reactions can run in supercritical CO₂.

   They combine gas and liquid properties, so they can host reactions that ordinary solvents cannot.

Common mistakes

• ✗ Writing that the particles of a solid do not move at all.

  ✓ They stay at fixed positions but oscillate about their mean positions.

• ✗ Saying plasma is rare everywhere because we seldom see it.

  ✓ It is not normally seen on Earth, but most of the matter in the rest of the universe exists as plasma.

• ✗ Calling a liquid crystal a kind of crystalline solid.

  ✓ It is an intermediate state: its properties lie between those of ordinary liquids and crystalline solids.

MCQs

1. 1. Matter in neon signs is in the state of:

   1. (a) Supercritical fluid
   2. (b) Liquid crystal
   3. (c) Gas
   4. (d) Plasma
   Show answer

   (d) Glowing signs and tubes contain a partially ionised gas, which is plasma; your book names fluorescent tubes as an example.

2. 2. A state of matter whose properties are between those of liquids and crystalline solids:

   1. (a) Supercritical fluid
   2. (b) Liquid crystal
   3. (c) Plasma
   4. (d) Dark matter
   Show answer

   (b) That is the book's definition of a liquid crystal. A supercritical fluid sits between liquid and gas instead.

3. 3. Which state is easiest to compress?

   1. (a) Gas
   2. (b) Liquid
   3. (c) Crystalline solid
   4. (d) Amorphous solid
   Show answer

   (a) Gas molecules are very far apart with very weak forces, so they are easily compressed.

4. 4. Which of these is an exotic state of matter?

   1. (a) Liquid crystal
   2. (b) Graphene
   3. (c) Bose-Einstein condensate
   4. (d) Supercritical fluid
   Show answer

   (c) The book lists Bose-Einstein condensate among exotic states. The other three are intermediate states.

5. 5. The particles of a solid:

   1. (a) move freely in all directions
   2. (b) oscillate about fixed mean positions
   3. (c) have no forces between them
   4. (d) are farther apart than in a gas
   Show answer

   (b) Strong attractions hold each particle in place, where it can only oscillate about its mean position.

Quick revision

• Solids, liquids and gases differ in particle forces, arrangement and distance; that sets compressibility and density.
• Plasma is a partially ionised gas, seen in fluorescent tubes, lightning and welding arcs.
• Supercritical fluids, liquid crystals and graphene are intermediate states; dark matter and BEC are exotic states.